Method for preparing a conductive coating on a glass surface

ABSTRACT

Method includes applying to a glass surface, such as the inner surface of an electron-tube glass envelope, at room temperature a coating of an aqueous stannous chloride solution and heating the coated glass in an air atmosphere to a temperature in the range of 275* to 450*C.

United-States Patent [191 Maley 3,713,884 Jan. 30, 1973 METHOD FORPREPARING A CONDUCTIVE COATING ON A CLASS SURFACE Inventor: James JohnMaley, Lancaster, Pa.

Assignee: RCA Corporation Filed: Sept. 25, 1970 Appl. No.: 75,552

U.S.CI. ..117/211, 117/335 C, 117/124B Int. Cl. ..IIlj 29/28, B44d 1/18Field of Search ..117/211, 124 A, 124 B, 201,

117/215, 159, 33.5 CF, 33.5 C

References Cited UNITED STATES PATENTS 10/1961 Payne et a1. ..117/2117/1967 Dates et al ..117/211 6/1964 Lineweaver ..117/211 X FOREIGNPATENTS OR APPLICATIONS 1,030,471 /1958 Germany 265.560 /1968 Austria..117/211 OTHER PUBLICATIONS Dalin et al. WADC Tech. Report" 53378,January 1954, page 23. 7

Primary Examiner-Ralph S. Kendall Attorney-Glenn H. Bruestle [57]ABSTRACT Method includes applying to a glass surface, such as the innersurface of an electron-tube glass envelope, at room temperature acoating of an aqueous .stannous chloride solution and heating the coatedglass in an air atmosphere to a temperature in the range of 275 to 4Claims, 2 Drawing Figures APPLY/N5 r0 4 GLASS "SURFACE/1746007 ROOM rmf/mru/rg A mam a OF M 400E005 sour/0m 0F sm/v/vous cam/05 HEAT/N6 THE60/175? 61/155 SURF/46f 70 A ffMPEfi/lH/Rf //V THE [WI/V65 OF 275% 450"6' llV/l/R This invention relates to a method for preparing an adherentconductive coating on a glass surface and particularly, but notexclusively, to a method for preparing an adherent conductive coating ona glass envelope of an electron tube.

In one method for preparing an adherent electrically conductivetin-oxide coating on a glass surface, described in U. S. Pat. No.2,118,795 to J. T. Littleton,

a solution of stannic chloride, hydrochloric acid and 1 water is appliedto a hot glass surface with the glass ata temperature usually about 600to 750C. Although this process provides suitable conductive coatings onglass, it is not suitable for coating of partially fabricated tubestructures and for uses in existing tube-fabricating processes. The hightemperatures may damage fabricated tube structure or distort the glass.

In another method for preparing an adherent conductive tin-oxide coatingon a glass surface, described in U. S. Pat. No. 3,005,731 to P. D.Payne, Jr., a solution of anhydrous stannous chloride (SnCl in anorganic solvent (not an aqueous solution) is coated on the glass surfaceat room temperature. Then, the coated glass is heated to at least 250C.This process is not economically practical for coating partiallyfabricated tube, structures since the use of an organic solvent requirestime-consuming solution preparation, elaborate safety precautions, andspecial processing equipment.

SUMMARY OF THE INVENTION The novel method includes applying a coating ofan aqueous solution of stannous chloride to a glass surface at aboutroom temperature. The coated glass is then heated in an air atmosphereto a temperature in the range of 275 to 450C.

The novel'method, unlike a prior process, uses an aqueous solution ofstannous chloride (SnCl,) applied to a glass surface at roomtemperature. The use of an aqueous solution of stannous chlorideeliminates the need for heating the glass to a temperature at whichfabricated tube structure may be damaged or at which the glass maydistort. The use of an aqueous solution of stannous chloride (SnClapplied to a glass surface at room temperature to obtain a conductivecoating is surprising since in one previous method a conductive coatingwas prepared by applying an aqueous solution of stannic chloride (snCland hydrochloric acid to a hot glass surface. The novel method alsodiffers from another prior process since an aqueous solution of stannouschloride (SnCl is applied to the glass surface. The use of an aqueoussolution of stannous chloride'eliminates timeconsuming solutionpreparation, the use of hazardous solvents, and permits the use ofexisting processing equipment. This is also surprising since in anotherprevious method it was necessary to apply a nonaqueous solution ofstannous chloride (SnCl,) and an 'organic solvent to glass at roomtemperature to obtain a conductive coating of stannous oxide.

'sion-picture-tube funnel BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is aflow diagram showing the steps of the I novel'inethod.

FIG. 2 is a sectional elevational view of a cathode-ray tubeillustrating the position of the conductive coating produced by thenovel method.

DESCRIPTION OF THE PREFERRED EMBODIMENT The flow-diagramof FIG. 1 showsthe principal steps of the novel method, which are (a) coating a glasssurface at room temperature with an aqueous stannouschloride solutionand then (b) heating the coated glass at a temperature of about 275 to450C in air.

' EXAMPLE 1 In one embodiment of the novel method, a solution isprepared by dissolving about 50 grams of anhydrous stannous chloride(SnCl in about 100 milliliters of water to produce a milky liquid. Theliquid solution is then coated on the glass inner surface of acolor-televiusing standard spraying techniques. In spraying, itis-preferred that the sprayed coating on the inner surface of the glassenvelope appear as fine drops of liquid. If too much stannouschloridecoating is applied, the coating may run,'and if too littlestannous-chloride coating is applied, the coating may not uniformlycoatthe glass surface. It is preferred that the coatingbe applied inmultiple layers to obtain the desired coating thickness. 'After thespray coating is applied, the coating may be allowed to air dry. Then,the glass funnel is heated in an air atmosphere to a temperature ofabout 350C for about 30 minutes. The coated glass is then graduallycooled.

' The novel process results in an approximate 10- micron-thick adherentconductive stannous-oxide coating on the glass surface having a surfaceresistivity of approximately 10,000 ohms per square.

EXAMPLE 2 The same procedure as described in EXAMPLE 1 is followedexcept the glass funnel is heated to 275C in an air atmosphere for atleast 1 hour. The resulting stannous-oxide coating has approximatelythe-same characteristics as the coating obtained in EXAMPLE 1 for thesame coating thickness.

EXAMPLE 3 A water solution of approximately 25 grams of stannouschloride (SnCI in about milliliters of water is applied on the innersurface of a glass-electron tube envelope to form a coatingapproximately 5 microns thick.'The glass envelope is then heated, in anair atmosphere toapproximately 275C held at 275C for about 30 minutes.The glass envelope is then cooled to room temperature, and the aforesaidprocess repeated a second time. The resulting stannous-oxide-c'oatingcharacteristics are approximately the same as obtained in EXAMPLES l and2 for the same final coating thickness. I

In the preferred method, a coating of an aqueous solution of stannouschloride is applied to a glass surface at room temperature. Previouslyin one prior process where a coating of an aqueous solution of stannicchloride and hydrochloric acid is applied to hot glass (600 to 700C),heating of the glass may damage the fabricated tube structure or distortthe glass and result in a defective tube. This prior process will notprovide a conductive coating when applied to a glass surface at roomtemperature. This prior process also uses a hydratedform of stannicchloride (SnClySl-QO) in the solution, where in the preferred methodanhydrous stannous chloride (SnCI is used.

Previously in another prior process where a coating of stannous chloride(SnCl and an organic solvent is applied to a glass 1 surface, thesolution must be prepared by rolling for approximately 8 hours to insurethat all the stannous chloride is dissolved, and then the solution mustbe filtered. The use of this solution requires special safetyprecautions which are not necessary with the aqueous stannous chloridesolution of the preferred method. The aqueous stannous chloride solutionmay also be processed with existing equipment of the type used forapplication of other conductive coatings such as graphite.

The coated glass is then heated ata temperature of about 275 to 450Cinair. Temperatures below 275C are insufficientto produce a conductivecoating. of stannousoxide. Temperatures above.450C produce conductivestannous oxide coatings but are undesirable for most tube-makingprocesses because they are not generally usedfand because they degradethe performance of other structures in the, tube. The duration ofheating is determined empirically. Generally, the higher the heatingtemperature, the shorter the required heatingtime.

The color-television-picture tube 10 illustrated in FIG. 2 comprises aglass envelope 11 having a funnel portion 12 and a neck portion 13. Anelectron-gun assembly 14 in the neck portion 13 is adapted to project anelectron beam through an apertured mask 15 incident upon a phosphorscreen 16 during the operation of the tube, The inner surface 17 of thefunnel portion 12 and neck portion 13 carries a conductive coating 18applied by the novel method.

Although acoating thickness -of approximately 10 microns is preferred,other thicknesses may be produced. Variations in solution concentrationsand coating thicknesses can be used to produce different conductivefilmresistivity values.

lclaim:

l. A method for preparing an adherent conductive film on a glass surfacecomprising the steps of l. applying to said glass surface at about roomtem perature a coating of a'solution of stannous chloride dissolved in asolvent consisting essentially of water 2. and heating said coated glassin an air atmosphere to a temperature in the range of 275 to 450C. 2.The method of claim 1 wherein said glass is heated in an air atmosphereto a temperatureof about 350C and then maintained at said "temperatureabout 30 minutes. I

3. The method of claim 1 wherein said glass is heated in an air'atmosphere to a temperature of about 275C and then maintained at saidtemperature about l hour.

4. The method according to claim 1 wherein the glass surface is theinner surface of a cathode-ray-tube envelope,. and step l)com rises:

coating a portion of he'mner surface of said envelope with said aqueoussolution of stannous chloride.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 5,713,884

DATED February 3, 1998 INVENTOR(S) Thomas W. Osborn, III, et al.

It is certified that error appears in the above-identified patent andthat said Letters Patent is hereby corrected as shown below:

Column 80, line 66, after panties" insert --and--.

Column 81, line 2, "external" should read extending--.

Signed and Sealed this Fifteenth Day of May, 2001 NICHOLAS P. GODICIAIIESIiHg Officer Acting Direcmr oft/1e Um'red Suites Purenr andTrademark Office

1. A method for preparing an adherent conductive film on a glass surfacecomprising the steps of
 1. applying to said glass surface at about roomtemperature a coating of a solution of stannous chloride dissolved in asolvent consisting essentially of water
 2. and heating said coated glassin an air atmosphere to a temperature in the range of 275* to 450*C. 2.The method of claim 1 wherein said glass is heated in an air atmosphereto a temperature of about 350*C and then maintained at said temperatureabout 30 minutes.
 3. The method of claim 1 wherein said glass is heatedin an air atmosphere to a temperature of about 275*C and then maintainedat said temperature about 1 hour.